Ukrainian troops attacked pro-Russia separatists Friday in the southern port of Mariupol, apparently driving them out of buildings they had occupied in the city.

About 100 soldiers emerged triumphant from the previously rebel-occupied buildings, shouting the name of their battalion, Azov, and singing the Ukrainian national anthem. They also destroyed an armored vehicle and a heavy truck used by the separatists, leaving the vehicles scorched and riddled with large-caliber bullet holes.

Mariupol is the second-largest city in the eastern Donetsk region, where armed separatists have declared independence from the government in Kiev. The Azov Sea port sits along the main road leading from Russia to the Crimean Peninsula, which Russia annexed in March from Ukraine.

Interior Minister Arsen Avakov said four government troops were wounded in what he called a successful operation. Witnesses said they saw troops capture at least four separatist fighters. There was no immediate word of casualties on the rebel side, and Associated Press journalists at the site were blocked from entering the buildings.

Ukraine and the West have accused Moscow of fomenting the unrest in eastern Ukraine and supporting the separatist fighters. Russia, however, has denied sending troops or weapons to Ukraine and has described the Russian citizens fighting with the separatists as volunteers.

The renewed fighting Friday came as rebel leaders confirmed they had obtained three tanks.

Government officials say the tanks were part of a column of armored vehicles that crossed the porous border into Ukraine from Russia.

Denis Pushilin, a leader of the separatist Donetsk People's Republic, told Russian state television Friday that they had the tanks but it was "improper to ask" where they had gotten them.

"They are in Donetsk and are the minimum that we have to defend the city," he said.

Both the U.S. and NATO condemned the reports of Russian tanks.

"In the last three days, a convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers and other military vehicles crossed from Russia into Ukraine near the Ukrainian town of Snizhne. This is unacceptable," U.S. State Department spokeswoman Mari Harf told reporters in Washington.

"I am concerned about reports that pro-Russian armed gangs are acquiring heavy weapons from Russia, including Russian tanks," NATO Secretary-General Anders Fogh Rasmussen said in a statement Friday. "This would mark a serious escalation of the crisis in eastern Ukraine."

Rasmussen urged Russia "to complete the withdrawal of its military forces on the border with Ukraine, to stop the flow of weapons and fighters across the border, and to exercise its influence among armed separatists to lay down their weapons and renounce violence."

The U.S. also urged Russia to back the peace plan laid out by new Ukrainian President Petro Poroshenko.

Russia has not commented on the tanks, but its border guard service said two Ukrainian armored vehicles crossed the border in the opposite direction Friday, into Russian territory near the southern city of Rostov-on-Don. Russian news agencies quoted a spokesman for the border guard service as saying that the Ukrainian troops were able to return to Ukraine but left behind one of the vehicles after it apparently broke down.

Pushilin repeated the separatists' call for Russia to send peacekeeping troops into eastern Ukraine. Russia has said this could only be done with U.N. authorization.

Poroshenko, who took office on June 7, rallied support for his plan to end the fighting in phone calls Thursday with Russian President Vladimir Putin and German Chancellor Angela Merkel. Poroshenko told Merkel he is willing to negotiate, but not with those he calls terrorists. He said he could offer amnesty only to people who don't have "blood on their hands."

According to his spokesman, Svyatoslav Tsegolko, Poroshenko told Putin it was "unacceptable" that tanks had crossed the border. A Kremlin statement said Poroshenko told Putin about his plan for resolving Ukraine's crisis.

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